1. Field of the Invention
The present invention relates to metal-ceramic composite substrates, and producing methods thereof. The invention also relates to brazing materials that can effctively be used in joining metals to ceramic substrates.
2. Description of the Prior Art
Several methods are known in the art of joining metal such as copper plates to ceramic substrates such as aluminum nitride (AlN) and two typical examples are the active metal brazing process described in Japanese Patent Laid-Open No.166165/1985 entitled "A method of Joining Nitride-Base Ceramics to Metals" and the process in which a copper plate is directly joined to a modified surface of an aluminum nitride substrate (as typically described in Japanese Patent Laid-Open No. 163093/1981).
The active metal brazing process provides a higher strength of joint than the direct joining process and the resulting joint has the desired charactristics such as high durability to repeated heat cycles. Hence, the active metal brazing process currently finds extensive use in the joining of copper plates to non-oxide-base ceramic substrates, say, nitride ceramic substrates.
Two brazing materials are commercially used in the practice of the active metal brazing process; one of them is composed of Ag, Cu and an active metal selected from among Ti, Zr and Hf (as described in Japanese Patent Laid-Open No.166165/1985) and the other is an active metal paste material composed of Ag, Cu and titanium hydride (as described in Japanese patent Laid-Open No.101153/1991).
Circuit substrates produced by joining copper plates to both sides of an aliminum nitride substrate with the aid of these brazing materials have already been commercialized.
In many brazing materials, an active metal is added to a structure comprising 72% of Ag and 28% of Cu, and accordingly the joining process is carried out at a temperature higher than the eutectic point of 780.degree. C., preferably, at a temperature of around 850.degree. C.
However, most recent circuit substrates are required to perform on larger electrical power and to meet this requirement, it has been desired to develop circuit substrates that are not only provided with good heat dissipating and electrical insulating properties but which also exhibit higher strength and thermal impact resistance.